Atmail is a fully-featured email server and Webmail client, allowing users to send and receive email via a Web browser or mobile device. It has full support for
IMAP mailboxes, and an optional email-server mode that uses EXIM as the MTA. Features include a scheduler, MySQL backend support, spell-check, address book, calendar user preferences, multiple accounts, SpamAssassin support, migration scripts from other mail servers, a customizable interface, an attractive AJAX interface, an optional group sharing Calendar/Addressbook, and an Outlook plugin to sync Contacts/Tasks/Calendar data between a Desktop, Mobile, and Webmail client or other email users.

Aliases2cdbm is a utility for converting mail aliases from a text file (e.g., /etc/aliases) into input suitable for the cdbmake utility. Cdbmake can then create a constant database (CDB) suitable for reliable, high-speed mail alias lookups.

Althea is an IMAP email client for X that uses the GTK widget set. It was built with the IMAP protocol in mind in order to avoid many of the pitfalls experienced by POP clients that added IMAP support. Althea allows easy access to a nearly unlimited number of servers, and has a very small memory footprint. As of version 0.5.0 Althea also supports IMAP over SSL.

AMaViS (A Mail Virus Scanner) scans e-mail
attachments for viruses using third-party virus
scanners available for UNIX environments. It
resides on a UNIX (Linux) machine and looks
through the attached files arriving via e-mail,
generates reports when a virus is found and sets
the delivery on hold.

amx takes a mailbox file and removes any messages which were received more than a given number of days ago. It also makes other configurable checks on the headers so you can ensure that unread messages or ones flagged as important can be kept.

anonvelope is a mail-message processing program that is designed to allow the message to make full use of Type-I (cpunk-style) remailers. It is especially designed to work with Pine, but it is possible it may be used with other MUAs.

Arrow is an elegant, powerful, graphical interface to electronic mail. This is not just another single window mail reader. Instead, it displays each mailbox and message in a separate window, thereby allowing one to simultaneously open as many mailboxes and view and compose as many messages as one wishes. This paradigm also allows one to drag-and-drop text between messages and organize ones mail by dragging messages between mailboxes.

AUC (Authenticated User Community) is an intranet system designed for use in an educational organization but is also useful in many other settings. It offers the ability for users to have a uniform web-based interface to discussion forums, e-mail (similar to hotmail, etc.), file management, and a searchable user database. Also, "Interactive Classrooms" provide a means for students and teachers to have a web-based extension to their in-class interaction. The system runs from a C-based monolithic CGI script. MySQL is used for database storage. Also, the web-based mail client supports MIME parts/attachments, IMAP, mbox, and multiple mail folders.

Autosig is a small utility for generating .signature files for email and .plan files for finger. The file is generated from a quote randomly selected from a file of quotes and a header file. The header file and the randomly selected quote are combined to form the signature file or plan file. It can be run as a frontend to your favorite mail client, or as a daemon.